Total capacities of and maximum numbers of users in strictly bandlimited CDMA and FDMA Gaussian channels

The author compares the total capacities of strictly bandlimited additive white Gaussian noise channels with CDMA (code division multiple access) and FDMA (frequency division multiple access) signaling. Theoretically, orthogonal signaling such as FDMA achieves the total capacity of a strictly bandlimited channel by using the sinc pulse with a `brickwall´ spectrum. However, when raised cosine pulses are used, it is found that maintaining orthogonality between users´ signals is no longer optimal and the total capacity of the CDMA channel is larger than that of the FDMA channel. The maximum numbers of users supported by CDMA and FDMA are compared, assuming that each user has the same available power and demands the same capacity. It is found that, in a strictly bandlimited channel, CDMA can support up to 70% more users than FDMA, depending on the rolloff factor, the symbol signal-to-noise ratio, and the number of users